Silicon semiconductor wafer solar cell and process for producing said wafer

ABSTRACT

A silicon semiconductor wafer is constructed from three mutually inclined monocrystalline regions (6, 7, 8) which form three circular sectors of the wafer whose interfaces and boundary lines consequently extend radially with respect to one another and form angles (W6, W7, W8) of less than 180° with one another. In this arrangement, two of the interfaces are first-order twin grain boundaries between two &lt;111&gt; crystal planes in each case. The silicon semiconductor wafer is used to produce inexpensive high-performance solar cells.

BACKGROUND OF THE INVENTION

For the photovoltaic generation of energy in the power range, highlyefficient, inexpensive, large-area solar cells with long-term stabilityare necessary which are composed of environmentally compatible materialswhich are adequately available. These requirements are not fulfilledsimultaneously at present by any solar-cell type. Monocrystallinesilicon solar cells (c--Si) which already fulfill all the otherrequirements except inexpensiveness come closest to the requirements atpresent.

The achievement of high efficiencies (>20%) requires, in addition to agood surface passivation (surface recombination rate Sr<100 cm/s for therear side and Sf<1000 cm/s for the front side or emitter side), the useof silicon crystals whose diffusion length L of the minority chargecarriers is about three times greater than the thickness d of theinitial wafer used. However, for reasons of mechanical robustness, the<100>-oriented c--Si wafers used at present still require a thickness ofover 300 μm. The diffusion lengths of approximately 900 μm necessary for20% efficiency can therefore be achieved only by the use of high-purity,low-oxygen and low-carbon crystals which are consequently veryexpensive.

The optimum physical thickness of c--Si solar cells is, however, not the300 μm used at present, but between 60 μm and 90 μm. This is due to thefact that, on the one hand, the open-circuit voltage Voc of solar cellsrises with decreasing thickness d, while, on the other hand, only asilicon layer thickness of approximately 100 μm is sufficient to absorbthe usable sunlight (AM1.5) completely and convert it into current. Inaddition, a cell thickness d of about 60 μm-90 μm necessitates asubstantially lower material requirement. As a result of the lowerrequirements imposed on the diffusion length (180 μm-270 μm), therequirements imposed on the material quality would also be lower, sothat inexpensive crucible-pulled material (Cz--Si) could also be used asstarting material for highly efficient silicon solar cells.

SUMMARY OF THE INVENTION

The Si monocrystals at present chiefly used as starting material are<100>-oriented and can be processed to form 60 μm-90 μm thick solarcells only with very high sawing and yield losses. An economicalproduction is therefore not possible on this basis.

One solution route for arriving at sufficiently thin, butfracture-resistant absorbers has been taken with polycrystalline siliconthin-layer solar cells on foreign substrates. The inexpensiveness,large-area producibility and productive capacity of this process havenot yet, however, been demonstrated.

The object of the present invention is to provide a solar cell made ofcrystalline silicon, which has, in a cost- and material-saving way, ahigh efficiency of 20 percent and over.

In general terms, the present invention is a solar cell constructed on amechanically robust 60 to 90 μm thick silicon semiconductor wafer as asubstrate. It has three mutually inclined monocrystalline regions whichform three circular sectors whose interfaces and boundary lines extendradially with respect to one another and form angles of less than 180°with one another. Two of the interfaces are first-order twin grainboundaries between two <111> crystal planes in each case. A lightp-doping is in the wafer with a shallow, n+-doped emitter 0.2-2 μm deepon a front side on the wafer and a first passivation layer on the frontside of the wafer.

A second passivation layer or a back surface field is on the rear sideof the wafer. Current-collecting contacts are on the front and rearsides.

Advantageous developments of the present invention are as follows.

The solar cell is produced from crucible-drawn silicon (Cz--Si), with anefficiency of more than 20 percent.

The interfaces are approximately perpendicular to the plane of the waferand form the angles W6, W7, and W8 with respect to one another, where:

W7=W8=(360°-W6)/2 and W6=109.47°±2°.

The surfaces of the three monocrystalline regions are <110> crystalplanes.

The solar cell has a texturing on the front side in the form of conesetched into the semiconductor surface.

The first passivation layer on the front side is an oxide layer whichproduces a charge carrier recombination rate Sf on the surface of lessthan 1000 cm/s.

The charge carrier recombination rate Sr on the rear side is less than100 cm/s.

The present invention is also a process for producing a siliconsemiconductor wafer for a solar cell substrate having three mutuallyinclined circular-sector-shaped monocrystalline regions. Threeoctahedral seed crystals are produced by sawing them out of aconventional <110>-oriented monocrystal such that all the octahedralsurfaces are identical with <111> crystal planes. Two of the seedcrystals are joined by laying them on top of one another and fixing themwith a wire so that a first-order twin grain boundary is formed. Usingthe two joined seed crystals, a bicrystal is grown by means of a crystalgrowing process. A wedge-shaped piece is sawn out of the bicrystal andin this process a <111> plane of the two half-crystals is laid bare ineach case. The third seed crystal is inserted into the wedge-shaped gapsuch that it forms a second first-order twin grain boundary with the<111> crystal surface. The bicrystal is shortened to approximately thelength of the third seed crystal to form a tricrystal seed. A tricrystalingot is pulled from a silicon melt by means of a crystal growingprocess using the tricrystal seed. Semiconductor wafers are sawn out ofthe tricrystal ingot using a wire saw.

Advantageous developments of this embodiment of the present inventionare as follows.

A molybdenum wire is used to fix the seed crystals.

A Czochralski process is used for crystal growing.

The tricrystal seed and the growing tricrystal ingot are aligned suchthat six mirror-like facets form on the tricrystal ingot.

After pulling a first tricrystal ingot from a quartz melting crucible,the silicon melt which is not more than half consumed is topped up byadding fresh silicon and the entire process is repeated up to ten times.

The sawn-off tip of a previously pulled tricrystal ingot is used as aseed crystal.

To solve the problem, therefore, a tricrystal is proposed which, withidentical electronic properties, has a substantially increasedmechanical robustness compared with conventional monocrystals, so thatself-supporting silicon semiconductor wafers according to the invention(tricrystal wafers) having a thickness of 60 μm-90 μm can still be sawnwith a very high yield of over 95% and further processed to form solarcells. Of comparably thin conventional monocrystal wafers, not a singleone survived the manufacturing process.

The basic advantage of the tricrystal according to the invention or ofthe silicon semiconductor wafer produced therefrom is that a crystalstructure is grown which contains no <111>-planes which run obliquelythrough the crystal and along which a silicon crystal normally firstundergoes dislocation and then breaks during the crystal pulling. Theinterfaces of the monocrystalline regions form angles of less than 180°with one another so that a straight continuous fracture formation isvirtually not possible and fracture formation along the angledinterfaces is made difficult.

This is achieved by producing a crystal structure in which the<111>-planes represent envelopes of the crystal and consequently extendexactly perpendicularly to the surface of the silicon semiconductorwafer. If tensile forces now occur along the surface which areperpendicular to a <111>-plane, no components of said force are producedparallel to the <111>-planes in this arrangement, with the result thatdislocation formation and a subsequent fracture are suppressed. If, onthe other hand, tensile forces occur which are parallel to a<111>-plane, the dislocation movement in the tricrystal is suppressed asa result of the fact that the tricrystal wafers do not contain anycommon continuous <111>-planes due to the tilting of the monocrystallineregions. A dislocation movement and fracture is blocked by the angledinterfaces.

The tricrystal described is disclosed in an article by G. Martinelli andR. Kibizov in Appl. Phys. Lett. 62 (25), 21 Jun. 1993, pages 3262 to3263. In the article, the posssibility is described of manufacturingwafers with a thickness of less than 200 μm from the tricrystal andusing them for photovoltaic applications.

Compared with the growing of monocrystals, the pulling of such atricrystal has cost, quality, and speed advantages which are of greatimportance for photovoltaic generation:

i.) With constant material quality, a tricrystal can be grown morerapidly by a factor of 2-3 than a <100>-monocrystal.

ii.) Without having to pull a cone tapering to a point, a tricrystal canbe pulled directly from the melt in Cz pulling. Because of the specialgeometry, no dislocations run back into the material in this case as inthe pulling of conventional monocrystal ingots (material and timesaving).

iii.) Because of the better mechanical robustness compared withmonocrystals, the cooling time of tricrystals can be reduced from 3hours to 1 hour without the ingot undergoing dislocation or shattering(time saving).

iv.) A repeat pulling of a plurality of ingots from the residual meltremaining in each case is possible only about twice in the case of amonocrystal because of dislocation formation as a result ofincorporation of impurities--in the case of tricrystals, pulling can becarried out about ten times as a result of the high resistance todislocation. As a result, a quartz crucible can be used repeatedly ifthe consumed silicon material of the melt is supplied again (multipleusage of the crucible, time saving).

v.) As a result of the possibility of pulling the crystal from the melt(ii.), of cooling the crystal rapidly (iii.) and of using a plurality ofcrucible charges (iv.), a rapid and quasi-continuous growth can beachieved. This makes possible, in particular in the case of shorttricrystal ingots, a very good uniformity of the crystal properties overthe ingot length since the melt is always removed by pulling only downto the same (small) portion.

Fresh silicon is replenished in a quasi-continuous process when thesilicon melt in the melting crucible has been consumed up to a maximumof one-half.

Because of the very good mechanical strength, tricrystal ingots can besawn using commercial wire saws with maximum yields to the optimumthickness for Si solar cells of 60-90 μm. These wafers are mechanicallyvery robust and can be processed further to form solar cells.

To produce solar cells having efficiencies of >20% on the basis of a 60μm-90 μm thick tricrystal wafer, a diffusion length L of the minoritycharge carriers of about 180 μm-270 μm is necessary. These values areachievable today even with inexpensive, crucible-pulled silicon (forexample Cz silicon), with the result that a high-performance cell canalso be produced with inexpensive material. For this purpose, a p-typetricrystal wafer (1-10 ohm*cm, L>180 μm) is provided with a 0.2-2 μmdeep emitter on one side by driving in phosphorus. The surfaceconcentration of phosphorus is in this case in the region ofapproximately 0.8-5×10¹⁹ cm⁻³.

In order to prevent the recombination of minority charge carriers at thesurfaces, the front side and the rear side of the solar cell have to bepassivated. For efficiencies of over 20%, a passivation of the frontside for recombination rates of Sf<1000 cm/s is necessary, while therear side must be passivated to Sr<100 cm/s. To passivate the frontside, an oxide can be used which protects the surface exposed to light.An antireflection layer composed, for example, of silicon nitride canavoid reflection losses and improve the efficiency of the solar cellfurther. The rear side can be passivated either by a "back-surfacefield" which is produced by driving in boron or by applying an oxide.

Since a complete collection of all the light-generated minority chargecarriers can already be achieved even with inexpensive starting materialas a result of the optimum thickness of the surface-passivatedtricrystal wafers, the type of construction used for the solar cellplays only a subordinate role if the shading and reflection losses arekept less than 8% and a fill factor of 80% is achieved. Such geometrieshave been proposed and already produced, for example, by R. A. Sinton,P. Verlinden, D. E. Kane and R. M. Swanson in Proc. of the 8thEC-PV-Solar Energy Coference, Florence, 1988, Kluwer AcademicPublishers, pages 1472 ff. and by M. A. Green in Proc. of the 10thEC-PV-Energy Conference, Lisbon, 1991, Kluwer Academic Publishers, pages250 ff.

The solar cell is contacted either by screen printing a silver pastethrough a mask with subsequent baking-in or by vapor deposition ofTi/Pd/Ag on the front side or of Al on the rear side of the cell.

Because of their crystal orientation, the tricrystal wafers cannot betextured with a texturing etc. Instead, use is made here of a standardcoating of the solar cell front side by applying titanium oxide.Alternatively, the solar cell front side can be textured with invertedcones in an inexpensive and rapid process. For this purpose, a resistwhich is resistant to silicon etchants is applied to the front side ofthe wafer, exposed to light in accordance with a regular hole patternand developed. The silicon surfaces laid bare in the hole pattern arefinally etched with an oxidizing acidic etching solution or a basicetching solution, in which process a hole pattern of inverted cones isproduced in the wafer surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several Figures of which like referencenumerals identify like elements, and in which:

FIGS. 1 to 6 show, in a diagrammatic representation, various processsteps in the production of a tricrystal ingot,

FIG. 7 shows a silicon semiconductor wafer according to the invention inplan view and

FIGS. 8 to 10 show the silicon semiconductor wafer in a diagrammaticcross section through various process steps in the further processing toform a solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Production of the seed crystal for tricrystal growing:

Referring to FIG. 1:

a) A conventionally produced and, for example, Cz-pulled, <110>-orientedmonocrystal is sawn with a diamond annular saw located in a goniometerwith an accuracy of 2° in such a way that an octahedron H having eight<111>-oriented surfaces is formed.

b) Step a) is repeated twice in order to produce a total of threeoctahedrons which are intended to form the seed crystals (H, T1, T2) forthe tricrystal.

c) Two of the octahedrons H, T1 from a) and b) are bound together with amolybdenum wire so that the first seed crystal (H) is joined to thesecond seed crystal (T1) in such a way that a first-order twin grainboundary is formed. It is also possible to fix the seed crystals H, T1to one another using other means, provided the material of the fixingagent is inert with respect to the silicon melt and has a higher meltingpoint than the latter. A tungsten wire, for example, would thereforealso be suitable. FIG. 2 shows the bicrystal seed BK produced in thisway in plan view.

d) From the bicrystal seed BK, a bicrystal ingot BS having a diameter ofabout 10 cm and a length of 10 cm is now grown by means of the Cz orfloating zone process (FZ) (see FIG. 3).

e) A wedge K is removed from the bicrystal ingot BS by means of agoniometer and a diamond saw in order to provide room for the third seedcrystal T2. For this purpose saw cuts are made along the 112 and the<114> direction (see FIG. 4) and a <111>-crystal plane is laid bare ineach of the two monocrystalline regions (H, T1).

f) The third seed crystal T2 is now inserted with Mo wire in such a waythat it again forms a first-order twin grain boundary together with theseed crystal H and with the crystal region grown therefrom (see FIG. 5).

g) The bicrystal ingot BS is now sawn back to the length of the thirdseed crystal T2. A tricrystal seed is obtained.

h) With the aid of the tricrystal seed from g), a thin tricrystal ingotabout 20 cm long which is no wider than 1 in² in cross section is nowdrawn by the Cz or FZ process.

Growth of tricrystals:

i) The thin tricrystal ingot from h) is used as seed crystal fortricrystal growing. In this connection, it is critical that, on the onehand, the seed crystal is oriented in such a way that the "seam" of thethree crystallites is oriented perpendicularly. On the other hand, acheck has to be made at the beginning of growth to see whether the sixmirror-like facets which are evidence of the production of thetricrystal are situated at the peripheries of the growing crystal ingot.If said facets do not appear, the growth must be started again.

In the crystal pulling process, the tricrystal grows at a rate which isapproximately 2-3 times greater than the growth rate of conventionalsilicon monocrystal ingots.

This is attributable to the increased number of 6 "growth surfaces" ofthe tricrystal.

As in the case of conventional monocrystals, the cross section of thetricrystal ingot depends on the pulling rate and can be adjusted to adesired value of approximately 6-8". The tricrystal is pulled to anydesired length which, without time or process disadvantages, can bechosen as substantially shorter than in the case of the conventionalpulling of monocrystals. In an advantageous way, the crystal ingotlength is made dependent on the size of the silicon melting crucibleused or vice versa. In order to avoid too sharp an increase inimpurities with growing length of the crystal ingot, pulling is carriedout until the silicon melt in the melting crucible, which isconventionally composed of quartz, has decreased by a third.

k) After successful growing, the tricrystal 3 is rapidly pulled out ofthe melt 4 and is allowed to stand for a few minutes about 2 cm above itso that a temperature equilibrium can be established. This processproduces a dislocation-free tricrystal (in this connection, see alsoFIG. 6).

FIG. 6 shows the pulled tricrystal in a diagrammatic representationduring the cooling phase. The tricrystal has an upper region 1, whichcorresponds to the thin tricrystal ingot produced in process step h). Inthe central region 2, the tricrystal ingot grows rapidly to the desireddiameter. This can take place substantially more rapidly than in thecase of conventional monocrystal ingots since no <111> crystal planesextend through the crystal transversely to the pulling direction, alongwhich planes dislocations may preferentially occur.

In the lower region 3, the tricrystal ingot already has the desiredcross section. Three of the 6 facets are indicated which form in areadily recognizable manner between the cut edges of the perpendicularlysituated <111> crystal planes. A substantial difference in thetricrystal ingot compared with monocrystal ingots is, furthermore, thatthe ingot terminates in a straight fashion at the lower end since it canbe rapidly pulled out of the melt 4. In the case of monocrystal ingots,on the other hand, a cone still has to be produced which is laterdiscarded again and therefore requires additional time and expense.

Quasi-continuous tricrystal growth:

l) After step k), the tricrystal is pulled right up in order to becooled to room temperature within about one hour in a protective gasatmosphere. The cooling time of only one hour (compared with 3 hoursotherwise necessary) also results in a further time and cost advantagesince, even with this high cooling rate, no stresses arise in thetricrystal ingot which could result in crack formation or other damageto the crystal ingot.

m) The hot quartz crucible 5, only about 1/3 (capacity about 30 kg)emptied by pulling, is refilled with about 10 kg of silicon. After aboutone hour, the silicon has melted and, during this time, the crystalingot from l) has also cooled.

n) The thin seed crystal 1 is cut from the crystal ingot 3 and reused instep i).

Since the crucible is only about 33% emptied by pulling, the crystalquality remains very homogeneous over the length in relation to the C,O, B, and P content of the tricrystal ingot.

Using the process according to the invention, up to approximately 10tricrystal ingots can be pulled from one and the same crucible 5 withoutthe crystal having to be emptied in the meantime or even having to bediscarded, as is necessary in the case of monocrystal ingots.

The quality of the 10th tricrystal ingot is then also still adequate tobe able to produce therefrom wafers for solar cells having highefficiency.

Using conventional wire saws, in particular using multi-wire saws,wafers of the desired thickness d are now sawn out of the tricrystalingot produced in this way. It emerges that the wafers can be handledeffortlessly even with a thickness of only 60 μm without unduly highfracture risk.

Using a conventional wire saw having a wire diameter of 300 μm andtherefore a material loss of 300 μm per saw cut, the sawing of a 60 μmthick wafer out of the tricrystal ingot requires an ingot length of 360μm. A monocrystalline wafer which can be handled only with a thicknessof approximately 330 μm requires, on the other hand, an ingot length of630 μm. Even this yields a material saving of approximately 40% with thethin tricrystal wafer.

Experiments have shown that the tricrystal ingot according to theinvention can also be sawn with thinner wires, with the result that thecut losses can be reduced further. With wires down to 80 μm thick, amaterial saving can be achieved in this connection of up to 75%. Thisresults in material costs per wafer which are reduced by a factor of 4.

FIG. 7 shows one of the silicon semiconductor wafers or tricrystalwafers according to the invention. The three monocrystalline regions 6,7, and 8, which originate from the three seed crystals H, T1, and T2,are of circular sector shape and form with one another or with respectto one another the angles W6, W7 and W8. If the octahedrons H, T1, andT2 used as seed crystals were sawn out exactly along the <111>-planes,and if the tricrystal ingot was pulled exactly vertically with sixregular facets and the wafers were sawn out perpendicularly to the ingotaxis, the angle W6 is exactly 109.47°. Given uniform growth, the twoother angles W7 and W8 are then exactly the same size and thereforeamount to 125.26°. Under the specified growth and sawing conditions, allthe three monocrystalline regions 6, 7, and 8 in the wafer have a<110>-surface or, to be precise, a <110> (6), a <0> (7), or a<0>-surface (8). It is, of course, also possible to saw out the waferswith the saw cuts not set vertically with respect to the ingot axis, inwhich case the wafer surface can be formed from other crystal planes andis correspondingly ellipsoidally shaped.

It was found that maintaining four boundary conditions is sufficient toproduce a solar cell having an efficiency of 20%. As already mentioned,these are the diffusion length L of the minority charge carriers and therecombination rates Sf and Sr of the front side and rear side,respectively. In addition, when the front contacts and the front-sidecoating or passivation are applied, care has to be taken that theshading and reflection losses remain below 8%, which is already achievedwith conventional processes. To maintain the first conditions L>3d whered=wafer thickness, diffusion lengths of L>210 μm are necessary for layerthicknesses of d=60 to 70 μm with the tricrystal wafers according to theinvention. This material quality can be provided with conventionalCzochralski-pulled standard silicon material (Cz--Si). In the case ofmonocrystalline wafers with d=300 μm, L>900 μm must be fulfilled. Thisquality is obtained only with a silicon material which is obtained froma floating zone process (FZ--Si). In this connection, the price ofFZ--Si is greater than that of Cz--Si by more than an order ofmagnitude. The boundary conditions Sf<1000 cm/s and Sr<100 cm/s can beachieved with conventional surface coatings. For this purpose, the frontside can be passivated with an oxide. For the rear side, it is possibleeither also to use a passivation oxide or to produce a back-surfacefield by doping with boron.

A further boundary condition, which is, however, largely dependent onthe boundary conditions already mentioned, relates to the fill factorFF, which should not be below 80%. This is also achievable with knownand tried processes.

Since alkaline texturing etching is not possible with the tricrystalwafers according to the invention because of their different crystalgeometry, a surface coating matched thereto is proposed, according tothe invention, to improve the light incident geometry. For this purpose,a hole pattern of inverted cones is etched into the wafer surface bymeans of a photoresist mask corresponding to this pattern.

Referring now to FIG. 8: A surface-wide protective layer is produced bysurface-wide screen printing of a photosensitive and, for example,positive-working and printable polymer 9 onto the front side of thetricrystal wafer 11 and baking out the polymer 9 at 150° C. forapproximately 10 s.

The polymer layer 9 is illuminated with UV light through a hole maskwith a given grid of holes each having a diameter of approximately 3-5μm in order to effect an increased solubility of the polymer in theirradiated regions 10 of the polymer 9 by photochemically producingpolar groups.

With reference to FIG. 9, the exposed polymer layer 9 can now be removedby wet-chemical etching, for example with a mixture of nitric,hydrofluoric, and acetic acid. An etch removal of the silicon situatedthereunder now begins simultaneously at these unprotected points 10.After approximately 10 seconds, inverted cones 12, which serve as lighttraps, form in the silicon surface.

The wafer 11 surface, now provided with inverted cones (depressions), islaid bare again by stripping the photoresist layer 9 with acetone orother solvents.

A further structural variant which is especially tailored for thetricrystal wafers according to the invention or the high-performancesolar cells produced therefrom relates to the arrangement of the frontcontacts.

Referring now to FIG. 10: It is proposed to apply the front contacts 13to mesa-like n++-doped semiconductor structures raised above theremaining wafer surface, while the remaining surface of the front sideis formed by the n+-doped emitter 16. To produce this structure, forexample, the emitter can be made deeper than usual by driving inphosphorus, for example to a depth of 1 μm. In a further doping step, ashallow n++-type doping, for example 0.4-0.8 μm deep, is driven into thewafer surface in a surface-wide manner. A photoresist procedure can beused to define the mesa structures 14, etching being carried out withthe aid of a photoresist mask and the n++-doped region being etched outwith the exception of the mesa structures 14 covered with thephotoresist mask. The front contact 13 is then applied over the mesastructures 14. It is also possible to apply the mesa structures 14together with the front contact 13 in a self-aligning manner. For thispurpose, the front contact 13 is first applied over the n++-type dopingand a subsequent etching step is carried out with the front contact 13,if necessary protected, as etching mask.

After producing the contacts, the passivation layer 15, which may be,for example, a grown-on oxide, can be applied.

The sequence and nature of the steps is, however, not critical for theproduction of a high-performance solar cell having an efficiency of atleast 20% if the abovementioned boundary conditions are maintained. Theform of the contacts on the front and rear sides is also of noimportance if the total shading and reflection losses remain below 8%.Thus, both contacts may be formed as point contact or as grid contactand the rear contact may additionally be designed as a surface-wideelectrode. In FIG. 10, the back contact 18 is designed as a printed-onand baked-in Al or Ag screen-printing paste over a passivation layer 19.It is also possible to produce a back-surface field in additionunderneath the passivation layer 19 by driving in boron in order toprevent the minority charge carriers (electrons) from diffusing to thesurface, where the recombination rate is increased because of the freedangling bonds.

A further advantage of the solar cells according to the inventionmanufactured from tricrystal wafers is their increased open-circuitvoltage, which in turn increases the filling factor and consequentlyalso the efficiency.

Consequently, higher-performance solar cells can be produced atsubstantially reduced costs using conventional and tried methods.

The invention is not limited to the particular details of the apparatusand method depicted and other modifications and applications arecontemplated. Certain other changes may be made in the above describedapparatus and method without departing from the true spirit and scope ofthe invention herein involved. It is intended, therefore, that thesubject matter in the above depiction shall be interpreted asillustrative and not in a limiting sense.

What is claimed is:
 1. A solar cell comprising: a mechanically robust 60μm to 90 μm thick silicon semiconductor wafer as a solar cell substrate,said wafer having a front side and a rear side, and said wafer havingthree mutually inclined monocrystalline regions which form threecircular sectors whose interfaces and boundary lines extend radiallywith respect to one another and form angles of less than 180° with oneanother, two of the interfaces being first-order twin grain boundariesbetween two <111> crystal planes in each case, the wafer being producedfrom crucible-drawn silicon;a light p-doping in the wafer; a shallow,n+-doped emitter 0.2 μm to 2 μm deep on the front side; a firstpassivation layer on the front side, the first passivation layer beingan oxide layer which produces a charge carrier recombination rate on thesurface of said wafer of less than 1000 cm/s; a second passivation layeror a back surface field on the rear side, the charge carrierrecombination rate on the rear side being less than 100 cm/s; andcurrent-collecting contacts respectively on the front and rear sides. 2.The solar cell as claimed in claim 1, wherein the solar cell has aconversion efficiency of at least 20 percent.
 3. The solar cell asclaimed in claim 1, wherein the interfaces are approximatelyperpendicular to the plane of the wafer and form angles W6, W7, and W8with respect to one another, where:W7=W8=(360°-W6)/2 and W6=109.47°±2°.4. The solar cell as claimed in claim 1, wherein the surfaces of thethree monocrystalline regions are <110> crystal planes.
 5. The solarcell as claimed in claim 1, wherein the solar cell has a texturing onthe front side in the form of cones etched into a surface of the wafer.6. A process for producing a silicon semiconductor wafer for a solarcell substrate having three mutually inclined circular-sector-shapedmonocrystalline regions, comprising the steps of:producing threeoctahedral seed crystals by sawing the crystals out of a conventional<110>-oriented monocrystal such that all octahedral surfaces areidentical with <111> crystal planes; joining two of the seed crystals bylaying the two seed crystals on top of one another and fixing the twoseed crystals with a wire so that a first-order twin grain boundary isformed; growing, utilizing the two joined seed crystals, a bicrystalusing a crystal growing process; sawing a wedge-shaped piece out of thebicrystal and in this process a <111> plane of the two half-crystals islaid bare in each case; inserting the third seed crystal into thewedge-shaped gap such that a second first-order twin grain boundary isformed with the <111> crystal surface; shortening the bicrystal toapproximately a length of the third seed crystal to form a tricrystalseed; pulling a tricrystal ingot from a silicon melt utilizing a crystalgrowing process using the tricrystal seed; and sawing the semiconductorwafer out of the tricrystal ingot using a wire saw.
 7. The process asclaimed in claim 6, wherein a molybdenum wire is used to fix the seedcrystals.
 8. The process as claimed in claim 6, wherein a Czochralskiprocess is used for crystal growing.
 9. The process as claimed in claim6, wherein the tricrystal seed and the growing tricrystal ingot arealigned such that six mirror-like facets form on the tricrystal ingot.10. The process as claimed in claim 6, wherein after pulling a firsttricrystal ingot from a quartz melting crucible, the silicon melt whichis not more than half consumed is topped up by adding fresh silicon andthe entire process is repeated up to ten times.
 11. The process asclaimed in claim 10, wherein a sawn-off tip of a previously pulledtricrystal ingot is used as a seed crystal.